This invention is directed to the manufacture of transparent, conductive indium tin oxide layers.
Thin film coatings which conduct electricity while simultaneously allowing transmission of visible light have numerous applications in areas such as flat panel displays, electromagnetic interference (EMI) and radio frequency interference (RFI) shielding. Several materials, such as tin doped indium oxide (In.sub.2 O.sub.2 /SnO.sub.2), known as indium tin oxide, possess a combination of electrical conductivity and visible light transmission. Currently, indium tin oxide, more commonly known as ITO, is the accepted material for liquid crystal display (LCD) applications, as well as EMI shields, due to its combination of low resistivity and high optical transmission. Furthermore, ITO is easily removed using etching techniques for circuit patterning. More precisely, It is known that a cover electrode for image sensor arrays must exhibit a high transparency on the order of at least 85% visible light and must simultaneously exhibit a low electrical sheet resistance of less than 300 ohms/square. Such specifications can only be met by indium-tin oxide (ITO) layers that have either been deposited at temperatures above 200.degree. C. or have been heat treated at temperatures above 440.degree. C. (see the report by Hamberg, et al., J. Appl. Phys. 60 (11) 1986, R 123 thru R 129).
A traditional process for depositing thin ITO films is the plasma sputtering of a Sn.sub.2 O.sub.3 doped In.sub.2 O.sub.3 sputter target at elevated temperatures in an oxygen environment. Temperatures must usually exceed 300.degree. C. during deposition to achieve an optically transparent electrically conducted film, but similar quality films can be achieved by heating the substrate after the material has been deposited. In either case, optical transmission as well as electrical conductivity are obtained by the addition of energetic oxygen to the environment. For example, one specific application of indium tin oxide coatings can be found in U.S. Pat. No. 5,704,837 directed to field emission devices with flat panel displays, disclosure of which is incorporated herein by reference. Unfortunately, temperature elevation during the treatment with energetic oxygen, can adversely affect the substrate physically.
Moreover, above 300.degree. C., the photoconductive, amorphous, hydrogen-containing silicon underneath the ITO layer loses its hydrogen content which is critical for its semi-conducting properties. The heat treatment at 440.degree. C. that could lead to meeting the required specifications in the case of cold-deposited and completely structured ITO layers can therefore not be applied (see the report by Raviendrad, et al., Physics Status Solidi (a) 88 (1985) K83 through K86, and the previously cited report by Hamberg, et al.).
In addition to the noted physical restrictions, heating and subsequent cooling of substrates in the course of manufacturing represents a considerable complication in view of the costs and the reduction in throughput times.
When the standard ITO manufacturing methods are modified to meet the restrictions, the specifications of the ITO are rendered noticeably poorer. The performance capability of the image sensors is thereby noticeably decreased.
In European Patent Application No. 0 217 095, to improve the values of resistance (300 ohms per square) with unaltered transparency (90%), heat treating is performed in two steps at a maximum of 200.degree. C., the first heat treating being carried out in an oxygen atmosphere for achieving the transparency and the secondly in a plasma-activated gas atmosphere.
Nonetheless, it would be desirable in this art to provide a method for forming a transparent coating of conductive indium tin oxide at relatively lower temperatures.